Marine Biology

Behavior of the Basking Shark



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Behavior of the Basking shark:



Whilst holidaying along the coast of Cornwall, some years ago now, my family and I decided to go for a stroll along the picturesque cliff tops around Sennen Cove. A little in to our walk I spotted what looked like two upturned surfboard fins, protruding above a dark mass in the water. Upon closer inspection we all realised it was a shark!
The two fins were approximately eight feet apart from dorsal fin to tail, and remembering what I had read in my 'encyclopaedia of sharks', I estimated that the beast was around thirteen feet in length.
Instead of panicking and shouting to the oblivious bathers, we watched in awe as the graceful creature effortlessly negotiated the shoals of fish and rocks, before growing bored of this impromptu show and disappearing back into the Atlantic.

We didn't need to warn the bathers because the shark in question was a Basking shark (Cetorhinus maximus). A welcome and frequent visitor to the coasts of Britain, and most coastal waters in a temperature range of 46 57 degrees Fahrenheit.
The shark is completely harmless to humans as it is a filter feeder, preferring to munch on 'zooplankton' (microscopic invertebrate larvae), by passively opening its mouth and filtering the tiny food that enter its huge gills.

Physiology:

The Basking shark is the second largest fish in the world, behind the Whale shark (Rhincodon typus), making it the second largest shark in the world.

Adults can reach up to ten metres or 33 feet in length, and weigh in at around eight tonnes. However these specimens are more likely the exception rather than the rule. In recent years fishermen and marine biologists have reported that most individuals are around eight metres or 26 feet in length and weigh around six tonnes.

As its name suggests, the Basking shark spends a portion of its time basking at the surface of the oceans where it is found, which include all the worlds major oceans laterally across the earth, determined by bands of temperature.

Geographical Location:

Typically these range from 46 to 57 degrees Fahrenheit, and include the waters in the west from Alaska to Oregon, east across the Atlantic to the Northern Pacific around Japan (and all waters in between). In Southern regions Basking sharks occupy the extreme southern oceans, preferring the colder temperatures, and can be found along the coasts of Argentina and Chile, eastward toward New Zealand.

Other aspects of Basking Shark behavior:

Despite belonging to the mackerel shark family or Lamniformes, whose other members include the Porbeagle (Lamna nasus) and even the Great White (Carcharodon carcharias), the Basking Shark has more in common with its vast cousin, the Whale Shark (Rhincodon typus). This is because both are huge fish, and both are filter feeders as oppose to their carnivorous relatives.

Although mainly a solitary animal, shoals of Basking sharks will congregate around large concentrations of plankton blooms. These usually occur around sunny areas where the plants photosynthesise and are in turn fed upon by zooplankton, which the shark also consumes. This is why individuals can often be seen 'basking' in the sunshine, as they harvest the bumper crop of plankton.
Indeed sometimes sharks can be observed swimming upside down as they feed, seemingly relaxing as they are too large for most predators to tackle them. Their extreme size is a defence mechanism, found in most large herbivores, such as the Whale shark, and land mammals such as Elephants.

When feeding, a Basking shark will open its huge mouth (around the height of a child) and filter the water as it passes through its industrial sized gills. As the water is expelled through these gills, mucus-laden gill rakers trap the plankton which the shark quickly swallows.

Studies have also shown that in winter, these sharks will dive up to 1000 metres, presumably to follow the plankton, which descends as the sun they use to photosynthesise, passes over the continent for another six months. This may also explain why it was thought Basking sharks were so elusive, as scientists hadn't realised that they were in fact pursuing their only source of food, rather than hibernating as was previously thought.

Despite being an extremely slow-moving shark when feeding, it is prone to bursts of speed when disturbed, and anecdotally has been reported to jump clean out of the water on occasion.

Although considered a deep-water shark, females will seek out shallower waters to give birth to their young in a descended yolk sac, like most sharks do.

Unfortunately the Basking shark is currently considered as 'Vulnerable' in the IUCN Red Data Book species list, which means it is likely to become 'Endangered' unless circumstances threatening its survival are improved. These include the over-farming of fish species, which in turn means more sharks are caught in nets and lines. Basking sharks themselves are also still hunted for their fish oil and supposed 'healing properties'.

Sources:

Burnie, D. (2001). Animals. Dorling Kindersley, London.

Server, L. (1990). The World of Nature: Sharks. Gallery books.

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